Whole human blood includes predominantly three types of specialized cells: red blood cells, white blood cells, and platelets. These cells are suspended in a complex aqueous solution of proteins and other chemicals called plasma. Although in the past blood transfusions have used whole blood, the current trend is to transfuse only those blood components required by a particular patient. This approach preserves the available blood supply and in many cases is better for the patient, since the patient is not exposed to unneeded blood components. Storage lifetimes can also be increased by packaging the individual blood products separately.
The blood components needed for the transfusion are taken from a donor by a process called apheresis in which the desired one, or more, specific components of the whole blood are separated and harvested by a blood-processing machine. The remaining components are returned to the donor. (As used herein, the term "donor" connotes anyone from whom blood is drawn for collection or processing, and can include volunteer donors or medical patients to whom blood collected components are returned.)
Using current methods, while only 10-12 minutes are required for a donor to donate whole blood, 30 minutes or longer may be necessary for donation of plasma or platelets by means of apheresis. As a result, the population willing to donate through apheresis is much smaller than the whole-blood donation population. This has become problematic as the need for plasma and platelets has greatly increased.
U.S. Pat. No. 5,387,187 discloses an apheresis method and apparatus that facilitates obtaining, from a single donor, standard units of blood components with hematocrit in the 65-70% range, and which is also capable of collecting volumes of plasma (with or without platelets) in the range of 400 ml in 20 minutes or less. The device provides a separation chamber, having input and output ports, for separating blood components into components according to their densities. The output port of the separation chamber is in fluid communication with a first container or containers that receive a less-dense component, while the input port is in fluid communication with a second container that receives more-dense components. A phlebotomy needle for withdrawing whole blood from a donor is in fluid communication with a third container containing anticoagulant.
In operation, a collection cycle begins with the withdrawal, through the phlebotomy needle, of whole blood from a donor. The whole blood is anticoagulated by mixing with anticoagulant drawn from the third container, and the anticoagulated whole blood enters the separation chamber through the input port. Lower-density components are separated from higher-density components in the separation chamber. The less-dense component(s) (e.g., plasma and platelets) are displaced through the output port into the first container or containers. The separation process is then terminated, and the higher-density components; (e.g., red blood cells or "RBC") remaining in the separation chamber are diluted with diluent and returned to the donor. More specifically, a diluent solution is stored in a fourth container in selective fluid communication with the flow path between the input port of the separation chamber and the phlebotomy needle, and the higher-density components remaining in the chamber are drawn out through the inlet port, mixed with diluent from the fourth container and returned to the donor via the phlebotomy needle.
In the second part of a collection cycle, whole blood is again drawn from the donor and combined with anticoagulant from the third container. The anticoagulated whole blood enters the separation chamber, which again separates the lower and higher density components. The second separation process is then terminated and the phlebotomy needle is removed from the donor. In this instance the higher-density components remaining in the separation chamber, instead of being returned to the donor, are displaced to a second container which is in selective fluid communication with the input port of the separation chamber. A fifth container containing a volume of additive solution is in fluid communication with the second container and rejuvenates the higher density components entering the second container with additive solution. Since donors can ordinarily part with greater volumes of plasma and platelets than RBC, this process facilitates simultaneous but separate collection of RBC and less-dense components such as plasma or plasma and platelets in proportions tolerable to typical donors.
While efficient and straightforwardly practiced, this approach nonetheless exhibits limitations imposed by the fixed nature of the various apparatus components. In particular, because the volume of the separation chamber cannot be altered, the overall amount of blood components collected during the course of a cycle is constant. If the target collection volume is not an integral multiple of the amount collected in the course of one cycle, procedure time will be increased and blood components needlessly withdrawn from the donor must be returned.
A number of considerations can influence target collection volumes. One key factor is donor physiology. Generally, it is desirable to obtain as much of a blood component as the donor is able to provide without risk of harm. The allowable volume varies from donor to donor, depending on such characteristics as the donor's weight, sex and the concentration of the desired blood component. With current systems, such as the RBC system described in the '187 patent, the operator first manually establishes a target collection volume for one or more blood components from a particular donor, and then computes the number of cycles necessary to obtain that volume. If, as is usually the case, exactly reaching the target collection volume would require a non-integral number of cycles the operator "rounds up" and performs the next higher integral number of cycles, returning to the donor the excess components from the separation chamber and the first collection container. This procedure is cumbersome, wasteful of time and needlessly prolongs the collection process.